Since transmission bandwidth is limited in a voice communication over mobile telephones, the quality of a received voice is significantly degraded from the quality of the voice actually spoken by a user.
To improve the quality of the received voice, conventional mobile telephones perform signal processing on the received voice, such as a filtering for adjusting the frequency spectrum of the voice.
Each user has his or her own unique feature in voice. If the received voice is subjected to a filtering operation having the same tap coefficient, the quality of the voice is not sufficiently improved depending on different voice frequency characteristics of users.